The present invention relates generally to sawmill carriages and in particular to end dogging sawmill carriages, including front and rear carriage units having dogs which engage the opposite ends of a log and a pair of independent drive means for such carriage units which are releasably connected together temporarily to maintain the log clamped between the carriage units while such pair of drive means both share the load and drive the log toward a primary breakdown log cutting means.
A dual carriage system, including two of such end dogging carriages can be provided on opposite sides of the index line of the same primary breakdown log cutting means for more efficient operation and higher lumber production. As a result, one carriage can be loaded with the next log while a first log is being transmitted by the other carriage through the cutting means such as chipping heads and saws to cut the log into lumber.
It has been previously proposed to provide an overhead end dogging sawmill carriage with a pair of carriage units, including a front unit and a rear unit which are driven by a common drive means such as a motor-driven cable as shown in European Patent Application Publication No. 0 195,423 of Foster, published Sept. 24, 1986, assigned to Kockums Cancar, Inc.; U.S. Pat. No. 4,697,487 of Cameron, issued Oct. 6, 1987; and U.S. Pat. No. 4,317,398 of Jones, et al., issued Mar. 2, 1982. The front and rear carriage units provide a pair of opposed dogs which engage the opposite ends of the log to clamp the log between such carriage units and carry the clamped log through the primary breakdown log cutting means when driven by their common drive means. The distance between the front and rear carriage units must be adjusted to accommodate logs of different length, and this has been accomplished through the use of a cylinder means as shown in the above patent application of Foster, a clamping cable per the above Cameron patent or a rack and pinion means as shown in the above Jones patent. This use of a common drive for both carriage units involves a cumbersome and heavy carriage structure requiring a large horsepower motor to drive the carriage, which is slow and expensive. In addition, it has been previously proposed in U.S. Pat. No. 3,503,428 of Ackerfeldt, issued Mar. 31, 1970, to provide a plurality of end dogging carriages for the same primary breakdown system in a sawmill which are all driven by a common drive means, requiring a heavy drive chain and a large motor that is even more expensive.
The end dogging sawmill carriage of the present invention differs from such prior end dogging carriages by employing two independent drive means for the front and rear carriage units so that they can be quickly and easily operated in different directions to change the distance between the end dogs for clamping logs of different length between the front and rear carriage units. Also, the two drive means can be temporarily connected together to share the load when driving the log through the cutting means. This allows the carriage units to be made of smaller size and lighter in weight so that the drive motors used for their independent drive means can be of smaller horsepower.
It has been proposed to operate two independent drives connected to the front and rear carriage units of an end dogging sawmill carriage in such a manner that the front carriage unit resists forward movement and exerts a pressure on the log in a rearward direction to maintain the log clamped between the front and rear carriage units. For example, see U.S. Pat. No. 4,338,986 of Detjen, issued July 13, I982, assigned to McDonough Manufacturing Company and the other Detjen patents cited therein. However, when the clamped log is moved forward in such a carriage, the drive means of the front carriage unit is not helping the drive means of the rear carriage unit drive the log forward but actually is hindering such forward drive by pushing the log rearward to maintain clamping. As a result, the rear carriage unit drive motor must be of even larger horsepower than a common drive means, since it must not only do all the work to drive the carriage forward to move the log through the cutting means, but must also overcome the rearward pressure of the front carriage unit drive motor. In addition, in some cases in order to maintain proper clamping pressure during movement of the log, the two independent drives must be synchronized to maintain the carriage units at the proper relative position and speed, which requires a complex electronic control system. Furthermore, if there is a power failure to the drives or the drives lose synchronism, the proper clamping of the log may be prevented so that the log is dropped from the carriage, resulting damage to machinery and a serious safety problem for operators within the mill.
The above-identified problems have been overcome by the present invention, which includes a releasable connection means for mechanically connecting the pair of independent drive means together temporarily while the log is clamped between the dogs of the front and rear carriage units, and for maintaining the log clamped while the log is driven forward toward the primary breakdown cutting means by both of such pair of drive means. Thus, by such releasable connection means, it is no longer necessary for the front carriage drive means to drive the front carriage unit rearwardly, thereby opposing the forward drive of the rear carriage unit. Instead both the front and rear carriage unit drives share the load and drive the log forward through the cutting means, thereby reducing the required size of the drive motors. In addition, there is no danger in the log falling from the carriage due to power failure or lack of synchronism, since the log clamping is maintained by the mechanical connection of the releasable connection means.
The sawmill carriage of the present invention in one embodiment temporarily connects the rear carriage unit to the drive cable of the front carriage unit by means of a cylinder actuated cable clamp or other automatically actuated mechanical connector so that the spacing between the carriage units can be adjusted to different log lengths and then fixed. Once the cable clamp is actuated, the carriage drives for the carriage units can both be used to drive the carriage forward to move the log through the primary breakdown cutting means. This results in several advantages, including a reduction in the size of the carriage drive motors because the two drive motors share the load evenly, both driving the log through the cutting means. Thus, the front carriage unit no longer needs to be driven rearwardly by its drive unit to maintain the log clamped between such carriage units. In addition, as discussed above, the log clamp pressure will not be lost in the event of a drive failure, which prevents accidental dropping of the log, thereby making the sawmill carriage safer. Furthermore, the carriage units and their drive systems can be of smaller size and of lighter weight construction, thereby improving the response time of the carriage and reducing the cost of the carriage. Another advantage is that since the front and rear carriage units are operated by independent drive means, they can be more quickly and easily adjusted to change the distance between the carriage units to accommodate a wide range of log lengths.